by Clark Davenport and Jose Sena
Vibrations caused by blasting operations can be annoying to individuals and detrimental to structures. Blasting vibrations are being measured at the Pueblo Viejo gold mine in the Dominican Republic in order to develop vibration control measures while maintaining mine production. Current blasting operations are being concentrated in a sensitive area of the zone, within 25 meters of the main agitator tanks. Vibration measurements are being utilized to develop attenuation curves and loading schedules such that ore processing operations will not be disrupted by blasting. Future mining operations will be expanded to other sensitive areas and vibration measurements will be utilized, with geologic and geotechnical information, to develop vibration risk maps and design vibration control measures. To increase tailings impoundment, the existing tailings dam is being raised. Geotechnical considerations dictated that portions of the existing tailings be densified. A test program of densification using explosives was performed. Vibration measurements were used to develop attenuation curves and loading schedules such that vibrations from the densification operations would not adversely effect the tailings dam.
The Pueblo Viejo gold and silver mine is located in the central portion of the Dominican Republic, near the town of Cotui (Figure 1). The Dominican Republic occupies the eastern two thirds of the West Indies island of Hispaniola, approximately 600 miles south of Miami, Florida. In 1992 the island, and in particular its capital of Santo Domingo, will celebrate the 500th anniversary of discovery by Cristobal Colon (Christopher Columbus). During his second voyage to the New World in 1494, Colon sent a small contingent of miners into the interior of the island in search of gold. These men most likely observed the crude placer-like workings of the Arawak Indians. Further exploration led to the discovery of promising gold deposits in the area of Pueblo Viejo. The first documented reference to Spanish mine workings in the area dates to 1505. Based on the abundance and size of the townsite ruins, it is believed that the mining activity was quite sizeable. Spanish mining activity continued until 1525; however reduction in the native work force, due to oppressive working conditions and disease, and much larger gold discoveries in other parts of the New World led to a cessation of mining. There are further references to the mine in 1690, 1825 and 1840, but after 1840, the property was apparently forgotten. In 1950, the government directed that the workings be evaluated for possible exploration effort. Between 1950 and 1953, exploration activities consisted of an extensive program of drilling, assaying and limited mining. The results of this exploration program led to the construction of a pilot plant for ore processing, however it was not economically possible to extract gold from the sulfides, and the project was abandoned. Private industry financed an exploration program between 1953 and 1967, with the results that Pueblo Viejo was "rediscovered". Another processing plant was constructed in 1973, and a mill constructed in 1975. In 1979 the Dominican government purchased 54% of the stock of the mine. Mining operations and technical assistance are currently being contracted by the government from Rosario Resources Corporation, a subsidiary of Amax, Incorporated.
Figure 1
The mine is located within a series of limestone and dolomitic hills having an average elevation of 300 meters above sea level. The Pueblo Viejo deposit is epithermal in nature, with the gold deposits being developed by the weathering of a gold-quartz-pyrophylite deposit formed in a small basin in the upper part of an old volcanic unit. The deposit contains no mineable vein orebodies, but contains numerous narrow vein like structures that are closely spaced and irregular. These structures are being mined by open pit operations. Gold production is obtained from these features in the Las Lagunas Formation, a pocket of metasediments. Since 1973 production has been maintained from oxides, however the oxide reserves are projected to be depleted in the next few years. Current production amounts to 9,300 metric tons of ore per day, resulting in a recovery of 472 ounces of gold and 1,866 ounces of silver daily. Upon depletion of the oxides, production operations will be shifted to the sulfides. In the interim, transition ore, a mix of oxides and sulfides, will be mined. Mining of the sulfides encompasses two problems:
Processing of sulfides will result in a greater volume of tailings than the processing of oxides.
Much of the sulfide ore is located near, or under, existing mine facilities.
The extraction of the sulfides near existing facilities will require control of blasting vibrations so as not to effect milling and processing facilities.
Energy left over from rock breaking processes by blasting is transmitted to surrounding areas as elastic waves. As these waves travel they displace particles in their paths, causing the particles to oscillate before returning to their original positions. The oscillations constitute vibrations. Special seismographs have been designed to measure the vibrations in terms of particle displacement, velocity or acceleration. In Europe and the United States, numerous studies have been performed to determine the relationship between damage caused to structures and the intensity of vibrations. A study conducted by the United States Bureau of Mines (reference 4) concludes that damage to a structure is most closely related to the particle velocity of a wave passing through the earth. Most of the scaling laws (equations) applicable to vibration studies are based on measurement of particle velocity. Scaling laws used in vibration studies relate particle velocity to both the distance (D, in feet or meters) from a blast and the charge weight (W, in pounds or kilograms) of explosives per delay used in the blast. Delays are interval timing devices used in blasting operations to distribute the total amount of explosives detonated in a given time. Delays are used to control vibrations and to increase fragmentation. In general, vibrations will increase in intensity when the charge weight per delay is increased, and decrease in intensity when the charge weight is decreased. The distance of a structure from a blast divided by the square root of the charge weight per delay is called the Scale Distance (SD). Vibration intensity is related to Scale Distance by a general equation:
V = H(SD)-b
where:
v = Peak particle velocity
H = Site constant
SD = Scale Distance = D/W
b = Site constant
Once the peak particle velocity is measured in the field, the above equation can be used to develop attenuation curves. A typical procedure to is obtain vibration measurements at difference locations throughout a site, using small test blasts. The data is plotted on log-log paper as Scale Distance versus Peak Particle Velocity (Figure 2). The constant H is determined by the intercept of Log H, and b is determined from the negative slope of the straight line. For areas of similar geology and blasting operations, the equation can then be used to determine, before blasting, the maximum charge weight per delay for a given vibration limit. The accepted vibration limit (peak particle velocity) for single story, wooden frame dwellings is 2 inches/second (ips), however there is no standard vibrational limit for other types of structures, such that a 1 ips limit is often used as a general guide, even for wooden structures in some cases. Most machinery and sensitive equipment will have vibration limits which can be obtained from the manufacturer.
Figure 2
A test program of tailings densification using explosives was conducted near the Mejita tailings dam in 1986. A vibration survey was conducted to develop attenuation curves for the explosives, such that vibrations would not effect the dam. A peak particle velocity of 0.3 inches per second for the embankment was selected by Rosario's consultants. Attenuation curves were developed using the densification test shots, mine production blasts and a vibratory compactor (roller). These curves were developed for the embankment, the rock surrounding the impoundment and the tailings. The survey was conducted using a Slope Indicator (SINCO) Model S-2 Vibration Monitoring Seismograph. This instrument has the capability of recording the output from two, three-component geophones which can be spaced up to 1,000 feet apart. All explosive shots were recorded on both geophones at varying distances from each shot, in order to obtain difference Scale Distance values. Seismograms recorded from the explosives shots and the vibratory compactor were analyzed for both amplitude and frequency content. All three components of each geophone were utilized in this analysis, as actual particle velocity is represented by a vector summation of all three components. The attenuation curve developed for the rock surrounding the impoundment is shown in Figure 3. The equation for this curve is:
V = 8.9(SD)-0.918
Figure 3
Figure 4 represents a plan view of the agitator tanks that are currently used in the ore processing. The agitator tanks are founded on the ore bearing Las Lagunas formation. The new sulfide processing complex is to be built approximately 50 meters to the west of the agitator tanks. In preparation for the new processing facility, material was excavated to a depth of 18 meters , beginning approximately 25 meters west of the agitator tanks. This excavation was accomplished with explosives; however the possible effects of vibrations on the agitator tanks dictated that some level of vibration control be exercised. During the course of the excavation operations a vibration monitoring seismograph was not available for use. A refraction seismograph was used to determine the compressional wave velocities and frequencies of the materials to be excavated, and to determine the frequencies of the agitator tanks. Compressional wave velocity information is important for determining the optimum type of explosives to use. Frequency information is important in that should blasting operations result in producing resonant frequencies in the agitator tanks, damage to the tanks may result not from vibrational effects, but from wave action within the tanks themselves. The compressional wave velocities of the materials to be excavated ranged from 3,700 to 5,400 feet per second, and had frequencies ranging from 72 to 77 Hertz (Hz). The frequency of the agitator tanks, when excited by a sledgehammer was 8 Hz. Most of the research done on vibration control recommends that in the absence of a vibration seismograph, a Scale Distance of 50 be used. Based on the seismic refraction surveying done in the area of the agitator tanks and the vibrations survey conducted at the Mejita tailings dam, the site constants H = 8.9 and b = 0.918, as determined for the tailings dam, were also used for the agitator tank study. The vibration intensity can then be calculated as:
v = 8.9(50)-0.918 or V = 0.25 inches/second*
* units of inches/second when distances are in feet and charge weight in pounds, in millimeters/second when using metric distances and weights. Figure 2 shows the attenuation curve plotted based on the above equation.
The peak particle velocity value of 0.25 ips appears to be conservative based on a similar study for water tanks, conducted by one of the authors. The vibration limit set on the water tanks was 1.4 inches/second. The water tanks remained full during all blasting operations, as did the agitator tanks at Pueblo Viejo. A charge weight per delay was calculated (using SD = 50) for different distances of the blasting from the agitator tanks. A sample of this loading schedule is as follows:
Distance (feet)
Pounds of Explosive/Delay
50
1
75
2.25
100
4
150
9
200
16
There is a fault zone located between the blast area and the agitator tanks (Figure 4), which served as a barrier to vibrations impingent on the agitator tanks. Measurements made of vibrations with the refraction seismograph indicated that the fault zone also changed the frequency content of the waves from 25 Hz on the blast side of the fault to 20 Hz on the agitator tank side of the fault. In order to further reduce vibrations, a single line of blast holes, nine meters in depth and spaced 1/2 meter apart, was detonated prior to main blast. This line of holes, called a pre-split line, served two purposes; 1) to act as the finished excavation line and 2) to reduce vibrations from the main blast by creating a buffer (large crack) between the main excavation and the agitator tanks. The pre-split holes were blasted using a relatively new technique, developed in South Africa and refined by Atlas Powder Company. This technique, called Air Shock Blasting, requires less charge per delay of explosives as compared to conventional presplitting operations. The Air Shock method utilizes an inflatable rubber plug at the top of the drill hole, and required no stemming material to tamp the charge. Stemming is placed only on top of the plug to hold it in place. When the explosive in the bottom of the presplit hole is detonated, a shock wave travels up the air column trapped in the blast hole, and is reflected back to the bottom of the hole by the inflatable plug. At the same time, another shock wave travels the short distance from the charge to the bottom of the hole, and is reflected upwards in the air column towards the plug. These reflected shock waves produces stresses in the surrounding rock that are reinforced by the expanding gases from the explosives, resulting in an even distribution of stress in the surrounding rock, causing hole to hole cracking.
Figure 4
Rosario Domimcana recently purchased a SINCO Model S-6 Vibration Monitoring Seismograph for full time use at the mine. At program of small test shots is being planned throughout different areas of the mine property in order to develop attenuation curves for different geologic conditions and rock types. These attenuation curves will be used to produce vibration risk maps and explosive loading schedules for the different areas in which vibrations may effect mine facilities. At the present time noise resulting from the blasting operations is not a factor, however the S-6 can be used to record air blast data if necessary. This data can be analyzed in order to design noise abatement measures.
Bollinger, G.A., 1971, Blast Vibration Analysis, Southern Illinois Press, Carbondale, Illinois.
Leet, L. Don, 1969, Vibrations from Construction Blasting, Hercules Powder Company, Wilmington, Delaware.
Leet, L. Don, 1971, Effects Produced by Blasting Rock, Hercules Powder Company, Wilmington, Delaware.
Nichols, Harry R., Johnson, Charles F., and Duvall, Wilbur I., 1971, Blasting Vibrations and Their Effem on Structures, Bulletin 656, Bureau of Mines, Washington, D.C.
Davenport, G. C., 1979, Factors Affecting Blasting Operations, Pit and Quarry Magazine, November, December, Chicago, Illinois.
TOP
Home | About Us | What's New | Products | Rentals | Second Hand | Case Histories | Search | Contact Us
